Electrical bridge network



May 29, 1951 Filed Oct. 25, 1948 J. W. WHITELEY ELECTRICAL BRIDGE NETWORK 3 Sheets-Sheet 1 3, WM @AM A/forn .5

May 29, 1951 J. w. WHITELEY ELECTRICAL BRIDGE NETWORK I5 Sheets-Sheet 2 Filed Oct. 25, 1948 :ulllrllli Ill ailllll May 29, 1951 J. w. WHITELEY smcmxcm. BRIDGE NETWORK Filed 001;. 25. 1948 3 Sheets-Sheet 5 INVENTOR IasEP/l M Wl-l/TELEY Aflvrn cg Patented May 29, 1951 ELECTRICAL BRIDGE NETWORK Joseph William Whiteley, London, England, as-

signor to A; C. Cossor Limited, London, England, a company of Great Britain Application October 25, 1948,-Serial No. 56,461 In Great Britain October 31, 1947 Claims.

..=-- The present invention relates to electrical bridge networks. 7

The measurement of many physical quantities ,by means of a varying electrical impedance connected in a bridge network is alread well known. Thus, for example, in order to measure the gas pressure in the cylinder of an internal c0mbus-- V 1 tion engine, a pressure-sensitive capacity pick-up ance being measured. Also, it is frequently advantageous or necessary to earth one terminal of this impedance. 'A' bridge system in which-a flexible cable, com- The principal object of the present invention is to provide an improved bridge network in which the aforesaid errors are substantially reduced.

prising a single conductor with an inner screen and an outer screen, is used for the connection of: measuring apparatus to a grounded remote impedance is already known.

-. It has been found that when a cable is used for connecting-into a bridge network an element whose impedance is to be measured appreciable errors in measurement may result if the cable is energising source is raised. Thus, in measuring 1 high frequency vibrations it is necessary to-apply aradio frequency carrier to the bridge and if the length of cable is not negligibly small in comparison with the carrier wavelength considerable errorsmay arise.

The chief source of these errors lies in the variations of the distributed capacitances in the cable.

The invention makes use of a known bridge network of the type comprising two closelycoupled inductive ratio arms. r p

A known advantage of bridge networks of this type is that they can be arranged in such a manner that when used to measure the impedance between-a first and a second terminal of a three-terminal mesh network, the impedances between the first and third, and the second and third terminals respectively of the mesh network have but little effect on the accuracy of measurements made by the bridge.

By arranging that the 'self-capacitances of a cable connector, for connecting an impedance element into a bridge network, are connected between the first and third and second and third terminals of a three-terminal mesh network having the impedance element connected between the first and second terminals thereof, the afore said known advantage can be obtained,

According to the present invention, therefore, an alternating current bridge network comprises two closely-coupled inductive ratio arms, a cable connector for connecting into a third arm of the bridge an element. whose impedance is to be measured, a'balancing impedance connected in the fourth arm of the bridge, terminals for applying an energising voltage to the bridge, and terminals for connecting a balance-indicating device to the bridge, the cable connector includingjtwo independently-screened conductors whose screens are connected to. one another whereby there is formed a'three-terminal mesh network whose terminals are points on the two conductors respec- Another source of error in such measurements l arisesin the construction of the pick-up devices. For example, capacit pick-ups are prone to error as a result of changes in the insulating materials ances between the individual electrodes and a screen associated with thepick-up. Again, in

making measurements on a rotating body, such {as an engine shaft, the connections between the pick-up device and the measuring apparatus generally include slip-rings and brushes. Insulation leakage and variable capacitances between adjacent slip-rings mayproduce serious errors.

tively and the saidconnected screens, and means being provided for connecting the three-terminal mesh network to the bridge in such a manner that variations in the impedances between the two conductors and the screens have substantially no effect on the accuracy of measurements made by means of the bridge.

According to a preferred form of the invention the cable connector has athird screen surrounding but insulated fromthesaid connected screens, and a connection between'the third screen and one of the conductors, whereby the impedance between the first said screens and the third screen is in parallel with the impedance between the said one of the conductors and its independent screen.

The invention also provides an alternating current bridge network comprising two ratio arms includin respectivelytwo closely-coupled inductance elements, a cable connector for connecting into a third arm of the bridge an element whose impedance is to be measured, a balancing impedance connected in the fourth arm of the bridge, and two pairs of terminals for connecting an energising source and a balance-indicating device respectively to the bridge, a first of the pairs of terminals being connected across the whole or a part of one of the ratio arms, the cable connector comprising two independently-screened conductors whose screens are connected to one another and to one terminal of said first pair, one conductor being connected to the other terminal of said first pair, and the arrangement being such that the impedance between the other conductor and its screen is effectively across one diagonal of the bridge.

Five embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figs. 1 and 2 are schematic diagrams of two examples respectively of bridge networks of the type specified, and

Figs. 3 to 7 are schematic diagrams of the five embodiments respectively, each of which includes a bridge network according to Figs. 1 or 2.

In Figure 1, two closely coupled inductive ratio arms l and 2 are employed to balance a variable balancing impedance Z2 against an impedance Z1 which it is desired to measure, The ratio arms I and 2 preferably consist of a bifilar winding on a high permeability magnetic core 3. Alternating current from a generator 4, operating at audio or radio frequency, is applied directly to the ratio arm l and is also applied via impedances Z1 and Z2 to the ratio arm 2, the direction of winding being such that the current'through a detector can be reduced to zero by adjustment of the impedance Z2. When suitable windings are employed for theiratio arms I and 2, this state of balance will occur when Z2 is approximately equal to Z1.

Whenit is desired that the bridge balance should correspond closely with equality between the impedance Z1 and Z2 2. compensating impedance may be inserted in series with impedance Z1 to balance the leakage impedance of the ratio arms. I

With the arrangement of Fig. 1 impedances Z3 and Z4 may be placed in shunt with the detector and the generator respectively without disturbin the balance of the bridge, although they .will affect the sensitivity with which the detector can respond to. small changes in either the impedance Z1 or the impedance Z2 when the bridge has been adjusted to balance. The arrangement of the three impedances Z1, Z2'and Z4v constitutes a three-terminal mesh network.

The bridge system of Figure 2 may be derived from that shown in Figure 1 by a simple interchange of the ratio arm 2 and the impedance Z2. The basic'p'rinciple of operation is the same in .both arrangements but certain advantages are obtainable with the system shown in Figure 2.

Thus, in using a bifilar winding for the ratio arms I and 2, there is a difficulty in providing suilicient insulation between the conductors of the windings since in the circuit of Figure. 1 the full generator voltage will be applied between bifilar windings are greatly reduced when the arrangement of Figure 2 is employed.

When a ground connection is applied to one terminal of the impedance to be measured Z1, as indicated in Figures 1 and 2, there is a diiiiculty in providing an electrostatic screen for the variable impedance Z2 in the case of the system of Figure 1 since a stray capacitance from the junction of impedances Z1 and Z2 to ground would cause an error. In the system of Figure 2 this error is avoided, the reason for this being that in the circuit of Figure 2 one terminal of impedance Z2 is connected to the generator, so that a capacitance from this point to ground is in shunt with the impedance Z2 and therefore is not liable to introduce an error, while the other terminal of Z2, which is connected to ratio arm 2, is approximately at ground potential when the bridge is balanced, so that a capacitance from this point to ground is in shunt with the impedance Z4 and therefore is not liable to introduce an error, while the other terminal of Z2, which is connected to ratio arm 2, i approximately at ground potential when the bridge is balanced, so that a capacitance from this point to ground will not appreciably disturb the balance.

In a bridge network as shown in Figure 2 it is sometimes advantageous to connect the detector 5 between the lower ends of the ratio arms instead of the upper ends as shown. When this alteration is made, the balance will not be completely independent of the impedance Z: but the errors caused by Z3 may be reduced by employing ratio arms which have a very low leakage impedance. In the network shown in Fig. 2 the balance of the bridge can be made to correspond closely with equality between the impedances Z1 and Z2 if a compensating impedance is inserted in series with the impedance Z1 to balance the leakage impedance of the ratio arms. When the detector 5 is connected across the lower ends of the ratio arms this compensating impedance is inserted in series with the impedance Z2.

Another variation of the basic circuit of Figure 2 has the advantage that the bridge balance corresponds closely with equality between the impedances Z1 and Z2 without requiring the addition'of a compensating impedance. This variation consists in connecting the detector 5 across the mid points of the ratio arms.

It is well known that, in general, one may interchange the detector and the generator in a bridge network without altering the basic principle of the circuit. In the present case one may effect this interchange in both figures. It is then found, however, that the circuit of Figure 2 loses some of its advantages over the circuit of Figure 1.

In Figure 3, the impedance Z1 to be measured is connected by means of a cable connector comprising two conductors l5 and I6, independently screened by two screens I1 and 18 respectively, to measuring apparatus comprising the tightlycoupled inductive radio arms I and 2, the variable balancing impedance Z2, the energising source 4 and the indicator 5.

The screens l1 and I8 are short-circuited to one another and to earth as shown, and in addition a connection is provided from these screens to the junction of the ratio arms I and 2. The impedances existing between the conductors l5 and I6 and their respective screens l1 and [8 are represented by Z2 and Z4.

It will be seen from the drawing that the impedance Z3 is in shunt with the indicator 5 and that the impedance Z4 is in shunt with the generator 4. Hence variations in Z3 and Z, have substantially no effect on the balance of the bridge.

In Figure 4, the impedance Z1 to be measured is connected by means of a flexible cable comprising the two conductors I5 and IS, the screens I1 and I8, and an outer screen I9 to the measurin'g apparatus consisting of the closely coupled inductive ratio arms I and 2, the variable balancing impedance Z2, and' the detector 5. In addition, terminals 6 and I are provided for connection tofthe generator 4 of alternating current at audiovor radio frequency, while a terminal 8 is connected to a screening box enclosing the generator 4. The generator terminals 6 and I are connected as shown to the outer screen I9 and the inner screens, II'and I8 which although still connected together are not earthed. The inner screens I1 and III are connected together at the ends of the cable as well as in the vicinity of the connection to the terminal I. Alternativel -the cable may be constructed so that the inner screens I1 and I8 lie in contact with each other throughout the length of the cable. It is important that the inner screens I1 and I8 should be well insulated from the conductors I5 and I6 and from the outer screen I9. portant that the inner screens I1 and I8 should 1 provide eifective electrostatic screening between each of the conductors I5 and I6, and the outer screen I9, while also screening the conductors I5 and IE- from each other. In addition, it is important-that the resistance of the screens I7 and ,IB, should be negligibly small. The outer screen I9 may consist of a metallic sheath or braiden'closing the conductors I5 and I6 togetherwith their screens I1 and I8. Alternatively} each conductor -I5 and I6 may be provided withan inner screen and an outer screen, in which case the two outer screens are connected in'parallel so as to be equivalent to the single outer screen I9 shown in Figure 4.

In the vicinity of the remote impedance Z1, the conductor. I 5 is connected to terminal 9 which is thelive terminal of impedance Z1; conductor I6 and outer screen I9 are both connected to -terminal II which is the ground terminal of impedance .Z1,.while the inner screens II and I8 detector 5; the conductor I5 is connected to a terminal I4 and to the ratio arm I; the inner screens I1 and I8 are connected to a terminal I3, to the detector 5 and to the ratio arms I and 2.

In the operation of a bridge system of this type, theapplication of a generator voltage between the terminals 6 and I will cause a voltage to appear between the terminals I0 and II, this voltage being generally unequal to the generator It is also imtween conductor the volta'geandliable to change in'magnitude and phase when variations occur in the impedance Z4. Similarly, the voltage between terminals III and II will be liable to change appreciably if the cable is subjected to mechanical strain or temperature change, since variations in the distributed capacitance between the inner screens I1 and I8 and the outer screen I9 will have a similar efiect as variations in the impedance Z4.

Terminals I0 and II correspond with the terminals of the generator 4 in Figure 1. From terminal III in Figure 4 a current flows via the inner screens I1 and I8 to the terminal I3. Part of this current flows through the ratio arm I to the terminal I4 and thence to terminal II via conductor I6- Another part of the current from terminal I3 flows through ratio arm 2 and impedance Z2 to terminal I2 and then via conductor I5 to terminal 9 and through impedance Z1 to terminal II. In addition, a current will flow through the distributed capacitance existing between conductor I6 and screen I8, but no current will flow in the distributed capacitance be- I5 and screen I"! when the bridge is balanced, provided that the reactance of the conductor I5 in its screen I! is negligibly small compared with the impedance Z1 and provided that the resistance of the inner screens IT and I8, in parallel, is negligibly small compared with the impedance Z4 in parallel With'the impedance existing between the inner screens (I1 and I8) and the outer screen I9 by virtue of distributed capacitance between them. Thus, when the bridge is balanced, no current will flow through the detector 5 and no current will flow through impedance Z3. 'Variations in the impedance Z3 therefore have no disturbing efiect on the balance condition. On the other hand, variations in the impedance Z4 have no effeet on the balance condition for the reason that they have an equal effect on the currents in impedances Z1 and Z2.

In some practical applications of bridge systems of this type, there islittle difliculty in obtaining a sufiiciently low resistance-in the inner screen circuit and in these cases it is generally immaterial at what point along the cable the connection of terminals 6 and 'I is made. The connection may then be made in the neighbourhood of terminals 8, I2, I3 and I4 so thatthe generator lead will be short. When the resistance of the inner screen circuit is appreciable, so that variations of the impedance Z4, or variations of impedances within the cable, begin to have aserious effect on the balance condition, then a considerable improvement is obtainable by movin the connections of terminals 6 and I to a position near to terminals I0 and I I. Alternatively, the generator may be applied directly across terminals ID and I I.

When the reactance of the conductor I5 is appreciable in comparison with the impedance Z1 it is found that the current in impedance Z2 reaches zero when the impedance Z2 is adjusted to a slightly different value from that which corresponds with zero current in the detector 5. Thus with one adjustment, the bridge is insensitive to variations of impedance Z3 and with the other adjustment the bridge is insensitive to variations of impedance Z4. Further means to avoid this eiiect will be later described.

As already described with reference to Figure 1 a compensating impedance may be inserted to make the bridge balance correspond closely with equality between impedances Z1 and Z2 so as to leakage inductance of the ratio arms.

balance the error caused by the leakage impecondition if the two concentric lines are balanced.

Thus the conductor l8 and screen l8 should be as nearly as possible identical with the conductor l and screen This object is readily secured in practice by using two equal lengths of concentric cable and twisting them together before applying the outer insulation and the grounded outer sheath l9.

With reference to Figure 5, the arrangement and operation are the same as described with reference to Figure 4 except that the relative positions of impedance Z2 and ratio arm 2 are interchanged. As a result of this change, the system of Figure 5 exhibits advantages over that of Figure' l as previously described with reference to Figure 2.

The arrangement shown in Figure 5 may be modified as already described with reference to Figure 2. Thus the detector 5 may be connected across the lower ends of the ratio arms or across their mid-points.

When it is desired to measure a simple twoterminal impedance such as Z1 alone, then the terminal ID in Figures 4 and 5 becomes redundant. In this case, no connection need be brought out from the screens I"! and I8 at the end of the cable adjoining the remote impedance Z1. It is however desirable that the conductor l5 should not be exposed to stray capacitance to ground. The screen I! should therefore project as far as possible up to the terminal 5 of impedance Z1.

In some applications, the systems described with reference to Figures 4 and 5 may prove unsatisfactory since it may be difficult to reduce the reactance of conductor l5 and the resistance of the screens 51 and I8 to suiliciently low values to obtain a desired high degree of immunity from the effects of variations in impedances Z3 and Z4 and the effects of changes of impedances within the cable. In order to overcome these difficulties, an arrangement such as that shown in Figure 6 may be employed. This arrangement includes means for avoiding the undesired effects caused by'the reactance of conductor l5. For this purpose, a condenser 20 is inserted in series with the conductor I5 so as to neutralise the reactive F voltage drop in conductor I5. A cc. denser 2| is also inserted in series with conduct:,-: l6 so that the bridge will be balanced when the impedance Z2 is equal to the impedance Z1.

In order to determine the required values of the capacitance of this condenser 20, one may measure the'impedance across terminals 2 and 55 by means of a voltmeter connected across terminals 9 and it While the detector 5 is short-circuited. The value of capacitance 25 may then be adjusted to give a minimum voltage. With this value inserted and the normal connections restored, one may adjust the value of the capacitance 2| until detector 5 indicates a balance with equal values for the impedances Z1 and Z2. In this way the conductor H3 is tuned to resonance with capacitance 25 while the capacitance 2| is made resonant with conductor l5 in series with the y pp ying this method it is practicable to increase the length of the cable to the region of 5% to of the wavelength of the generator.

Alternatively, condensers 26 and 2| may have equal capacitance so as to resonate the reactance of conductors i5 and 5 while a compenlit) sating impedance is inserted to balance the leakage impedance of the ratio arms. When the detector 5 is connected across the upper ends of the ratio arms, as shown in Figure 6, this compensating impedance may be inserted between terminal l2 and the condenser 20. When the detector 5 is connected across the lower ends or the ratio arms the compensating impedance may be inserted between terminal l4 and the junction of ratio arm with detector 5.

When the condenser 2| is inserted in the remote termination of conductor is, as indicated in Figure 6, it is possible to adjust the circuit so that the value of impedance Z2 will indicate the true value of impedance Z1 with a high degree of accuracy over a wide range; It is generally more convenient, however, to insert the condenser 2| in series with conductor IS in the vicinity of the terminal M, the result being that the balance condition requires a constant ratio between the values of the impedance Z1 and Z2.

A further embodiment of the invention is shown in Figure 7 in which 38 represents a capacity pick-up mounted on a rotating shaft (not shown) The pick-up 38 has one terminal connected to ground via the shaft and again via a slip-ring 35 and a brush connection to the remote end of the outer screen IQ of a flexible cable. The live terminal of the pick-up 38 is connected via a screened lead 36 to a slip-ring 33 which is connected by a brush to the remote end of conductor l5. Lead 36 is provided with a screen 31 which is connected to rings 32 and 3-1 in parallel. The rings 32 and 34 are placed adjacent to ring 33 so that direct capacitance and insulation leakage from ring 33 to ring 35 or to ground, is avoided. The rings 32 and 34 thus serve as guard electrodes and are connected via brushes to inner screens I1 and i8 in parallel. The impedance existing between ring 33 and the two guard rings thus takes the place of the impedance Z3 of Figures 5 and 6, while the impedance between the guard rings and ground replaces the impedance Z4.

Conductor I5 is connected via terminal l2 to a condenser in series with ratio arm 2. Conductor I5 is connected via terminal l4 through a condenser 2 l, a resistance 52, and an inductance 53 to ratio arm I. In place of the variable balancing impedance Z2 in Figures 5 and 6, variable condenser 22 is connected in shunt with a fixed condenser such as 23, 24 or selected by a switch 25. The detector circuit is connected between the junction of ratio arm 2 with condenser 22 and the junction of ratio arm I with inductance 53. Thus the detector is connected across the lower ends of the ratio arms. In the present example, a screened lead 54, 55 is employed to feed a differential output transformer comprising a primary winding 45, and two secondary windings 45 and 41 on a high permeability magnetic core 48. Preferably, a transformer for this purpose is constructed by first twisting three lengths of insulated wire together and then winding them to form a coil. In this way the secondary windings and H are equally coupled to the primary 45 so that no voltage will appear across a gain control 49 connected across the lower ends of windings 46 and 41 when there is no voltage between conductors 54 and 55, notwithstanding that there may be a considerable voltage across the primary 45. From the gain control 49 an output signal is taken via terminals 55, 5| to an amplifier (not shown). Thus the terminal 50 may be connected to the grid of an amplifier valve while terminal may be connected to the cathode. Alternatively, such an amplifier may be connected to the screened output lead 54, 55 through a screened transformer of known type.

A resistance 42 is connected between terminal [2 and the junction of condensers 43 and 44 so as to provide a means of balancing a small quadrature voltage which appears between terminals l2 and I3 as a result of theresistance of the inner screens I! and 18. Alternatively, a variable resistance could be employed, and could be connected, for example, across terminals 12 and I4. Adjustment of condenser 44 also provides a convenient means for balancing the bridge when the power factor of th pick-up 38 is different from the power factor of the balancing condenser 22 together with fixed condenser 23, 24 or 25. v

Condensers and 2| are chosen to resonate the reactance of conductors I5 and I6 as already described with reference to Figure 6. The resistance 52 and inductance 53 are adjusted to balance out the leakage inductance and resistance of the ratio arms so that the value of capacitance 22 required to balance the bridge is accurately proportional to the capacitance of the pick-up 38 over a wide range.

In'order to provide a means for calibrating small changes of capacitance in the pick-up a variable condenser 29 is connected via a switch to a tapping such as 3| on a winding 4! which may be wound on the same magnetic core 3 as the ratio arms I and 2. Alternatively, a separate magnetic core may be used. Switch 30 is employed as a range multiplier for the variable condenser 29.

Terminals 6 and l are provided for the connection of a generator. These terminals are connected via conductor 39 and grounded screen to the inner screens l1 and I8 and the outer screen l9.

Typical values of components for use with this embodiment when the capacitance 38 lies between 100-800 pF are as follows:

Capacitors Resistors Transformer 8 20-27,000pF. 2l27,000pF. 22-200pF ariable) 23200pF. 24400pF. 25600pF.

2950pF (variable) 48-820pF. 44100pF.

50 turns bifilarly wound on a Mumetal core R. O. S. C. No. 423 of TOM 401 lamina- Inductances tions .004 thick.

53-l;rH. Transformer 48 Windings 45, 46, 47 are each of 140 turns on a similar core. In transformers 3 and 48 Wires are twisted together before winding.

These values are for a generator frequency of 300 kc./s. using a foot length cable having the following characteristics:

Total capacitance between each inner cono Wlndings 1, 2 and 41 are each of I claim:

1. An alternating current bridge network com,- prising two terminals and a cable connector for connecting into a first arm of the bridge an element whose impedance is to be measured, two closely-coupled inductive ratio arms having one end of each connected to said terminals respectively, a fourth arm connected between the other ends of said ratio arms, said fourth arm in eluding a balancing impedance, terminals for connecting a balance-indicating device between said ratio arms, said cable connector comprising two conductors connected respectively to the first said terminals, and two screens independently screening said conductors, means for connecting said screens to one another and to the junction of one of said ratio arms" with said fourth arm, and terminals for connecting an energising source to one of said conductors and said screens.

2. An alternating current bridge network comprising two closely' coup'led inductance elements connected respectively in two ratio arms of the bridge, a balancing impedance connected ina third arm of the bridge, a first pair of terminals for the fourth arm of the bridge, the terminals of said first pair being connected to said ratio arms respectively, a cable connector for connecting to said first pair of terminals an element whose impedance is to be measured, second and third pairs of terminals for connecting an energising source and a balance-indicating device respectively to the bridge, one of the last said pairs of terminals being connected across at least a part of one of said ratio arms, and said second and third pairs of terminals having a common terminal and said cable connector comprising two conductors and independent electrostatic screens respectively surrounding said conductors, means for connecting said screens to one another and to the common terminal of said second and third pairs of terminals and two capacitors connected respectively in series with said conductors for counteracting the inductive reactance thereof.

3. An alternating current bridge network comprising two closely-coupled inductance elements connected respectively in two ratio arms of the bridge, a balancing impedance connected in a third arm of the bridge, a first pair of terminals for the fourth arm of the bridge, the terminals of said first pair being connected to said ratio arms respectively, a cable connector for connecting to said first pair of terminals an element whose impedance is to be measured, second and third pairs of terminals for connecting an energising source and a balance-indicating device respectively to the bridge, one of the last said pairs of terminals being connected across at least a part of one of said ratio arms, said second and third pairs of terminals having a common terminal, and said cable connector comprising two conductors, independent inner electrostatic screens respectively surrounding said conductors, and an outer electrostatic screen surrounding said conductors, means for connecting said independent screens to one another and to said common terminal of said second and third pairs of terminals, means connecting said outer screen of said cable connector to one of said conductors, and a correcting network connected between the first said pair of terminals for counteracting the effects of the resistance of said inner screens.

4. An alternating current bridge network for measuring the impedance of one branch Z1 of a three-terminal mesh network having-two further branches Z3 and Z4, comprising two terminals for connecting said branch Z1 into a first arm of the bridge, two closely-coupled ratio arms having one end of each connected to said terminals respectively, a fourth arm connected between the other ends of said ratio arms, said fourth arm comprising a balancing impedance Z2, terminals for connecting an energizing source effectively across one diagonal of the bridge, and terminals for connecting a balance-indicating device across the other diagonal of the bridge,said energizing source and balance-indicating device having a common terminal for connection to the junction of said branches Z3 and Z4 of said three-terminal mesh network.

5. An alternating current bridge network comprising two terminals and a cable connector for connecting into a first arm of the bridge an element whose impedance is to be measured, two closely-coupled inductive ratio arms havin one en of each connected to said terminals respectively, a fourth arm connected between the other ends of said ratio arms, said fourth arm including a balancing impedance, terminals for connecting a balance-indicating device between said ratio REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Country Date Great Britain Oct. 3, 1946 Number 

